Trimethylsilyl-Substituted Propyne Iminium Salts as Building Blocks in [4+2] Cycloaddition Reactions

Abstract

3-(Trimethylsilyl)propyne iminium salts 6a-c and 2-(trimethylsilyl)ethynyl-1,3-benzothiazolium triflate 12 are suitable dienophiles for Diels-Alder reactions with buta-1,3-dienes, cyclopentadiene, cyclohexa-1,3-diene, and anthracene. Tri-tert-butylazete reacts with propyne iminium salts 6a,b to form Dewar­pyridines 15a,b. Thermal isomerization of 15b generates the sterically overcrowded pentasubstituted pyridine 16 which features a boat-shaped pyridine ring with the highest deviation from planarity so far reported.

Key words

alkynes - iminium salts - Diels-Alder reactions - azacyclobutadiene - non-planar pyridine

References

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9

According to PM3 calculations, the orbital coefficient in the LUMO of 6b is higher at the SiMe₃ substituted carbon atom of the triple bond (-0.32 vs. 0.05). In the HOMO of isoprene, the orbital coefficient at C-1 is larger than at C-4 (0.59 vs. -0.45).

19

Crystallographic data (excluding structure factors) for the structures of 10a and 16 have been deposited with the Cambridge Crystallographic Data Centre [CCDC-173956(10a) and -173955(16)]. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [UK. Fax: [phone]; E-mail: [e-mail]).